This invention relates to an apparatus and method for extracting power from water waves, particularly ocean waves.
Ocean waves represent a significant energy resource. It is known to use a Wave Energy Converter to extract power from such waves. Known Wave Energy Converters tend to be expensive, and have limited prospects for survival in extreme conditions. It is therefore an object of the invention to provide a Wave Energy Converter which obviates or mitigates the above discussed problems by providing a Wave Energy Converter with low capital cost, and improved prospects for survival in extreme conditions, for a given power output.
According to the present invention there is provided an apparatus for extracting power from waves comprising at least two body members and a coupling member wherein the coupling member is adapted to link adjacent ends of the body members in such a way as to permit relative movement of said body members.
Preferably the apparatus comprises a plurality of body members and associated coupling members wherein said coupling members are adapted to connect adjacent ends of neighbouring body members to form an articulated structure. Said structure is typically a chain. Said chain is preferably substantially straight. Said chain may be curved. Said chain may be zig-zag.
Preferably the apparatus is adapted to be situated in waves. Typically the apparatus is adapted to react against oncoming waves to absorb power.
The chain is preferably of length comparable with the longest wavelength against which the apparatus reacts to absorb power. The chain is preferably significantly shorter than storm wavelengths.
The apparatus is preferably self-referencing. For this purpose the apparatus may be configured such that, in use, the chain is positioned to span at least two wave crests. Preferably the apparatus is configured such that, in use, the chain is free-floating slack-moored and referenced against itself.
The apparatus may further comprise a mooring system. The mooring system may position the chain in a preferred site in the ocean. Typically the mooring system is adapted to orientate the chain in a preferred orientation relative to oncoming waves.
Preferably the mooring system is adapted to orientate the chain such that it spans at least two wave crests. Typically the angle of orientation of the chain to the mean wave direction (the yaw angle) may be varied to maximise power extraction. The mooring system may be adapted to provide restraint or excitation to the apparatus for the purpose of modifying its overall response.
The body members may be of any size. The body members may be of any shape. Preferably the body members are substantially cylindrical. Typically, to limit loading, the body members have sufficiently small depth and freeboard to experience complete submergence and emergence in large waves. That is, the chain may be configured to encourage hydrostatic clipping in extreme conditions.
The cross section of said body members may be substantially elliptical or oval with the larger dimension oriented substantially horizontally to allow a larger water-plane area, while still keeping the depth and freeboard of, said member sufficiently small to permit hydrostatic clipping.
Alternatively the body members may be of arbitrary cross-section to confer other characteristics to the overall response of the device. In a preferred embodiment said cross section is wedge shaped so as to cause a lateral (perpendicular to chain axis) shift of the centre-of-buoyancy of said member as it varies its degree of submergence to alter the roll response of the system to provide improved power extraction or survival. Body members of any cross section may be provided with fins, bilge keels or other protrusions to add hydrodynamic damping to any direction of motion desired. Bilge keels may be added to influence damping of motion along the axis of the chain to influence mooring response.
The ends of the front and rear body members may be shaped to influence hydrodynamic characteristics.
Preferably the front unit may be provided with a conical front end to minimise drag in extreme seas while the rear unit has a flat rear end to increase damping along the axis of the chain structure to add damping to the mooring response.
Preferably said body members incorporate areas of sacrificial structure that will allow very large joint angles before the overall structural integrity or flotation of the member is compromised. Said areas of sacrificial structure behave in a manner similar to the crumple zone on a car.
Preferably the coupling members are single degree of freedom joints. Alternatively the coupling members may be universal joints.
Preferably the joints are orientated in varying directions. More preferably each joint is orientated substantially orthogonal to an adjacent joint and substantially perpendicular to the main longitudinal axis of the chain structure.
The apparatus is preferably configured to apply a roll angle away from horizontal and vertical (a roll bias angle) to the joints of the chain. Said roll bias angle may be different for each pair of joints. Said roll bias angle may be varied to maximise power extraction. The purpose of the roll bias angle is to ensure that the apparatus is orientated at an angle such that its mode of motion in the orientation at that angle is resonant with incoming waves.
Typically the apparatus comprises elements adapted to resist the relative movement of said body members. Said elements may be adapted to extract power from said relative movement. Said elements may be springs. Additionally or alternatively said elements may be power extraction systems. Different magnitudes of constraint may be applied to the substantially perpendicular pairs of joints in order to induce a cross-coupled response. The ratio of the magnitudes of constraint may be used to increase power capture in small waves.
Typically the apparatus is configured such that its capacity to absorb power is controlled by the orientation of the joints and the differential restraint thereof.
The chain may be adapted to be buoyant. More preferably the chain is adapted to be essentially free floating.
The apparatus may be provided with a ballasting system. Said system may be variable. Said system may be actively or passively variable. Typically said variable ballasting system comprises ballast tanks comprising inlet means and outlet means, wherein said inlet means is larger than said outlet means.
Said variable ballasting system may be confined to a front unit or units of the chain so as, in use, to encourage the front of the chain to dive under large wave crests. Said variable ballasting system may act to vary the roll bias angle of the chain, for example by the provision of asymmetric ballasting.
Preferably the arrangement of the body members and joints which comprise the chain is adapted to suit a specific site. Preferably the length of the chain is determined by the wavelength expected at a specific site. More preferably the length of individual body members which comprise the chain is determined by the wavelength expected at a specific site.
Typically the apparatus is configured to maximise its capacity to extract power from a given sea state, but also to ensure survival in extreme conditions. More preferably the apparatus is configured to be effective at reacting only against waves of wavelength below a designated length.
Preferably the apparatus is adapted directly to convert the power absorbed from the motion of adjacent segments into electricity. Alternatively the apparatus may be adapted to tore the power absorbed for future use.
Further according to the present invention there is provided a method or extracting power from waves comprising the steps of:
deploying an apparatus comprising a chain comprising a plurality of body members coupled with joints orientated in different directions in such a way as to permit relative movement of said body members in waves;
orientating the chain""such that it spans at least two wave crests;
applying a roll bias angle to said joints;
applying different constraints to each direction to induce a cross-coupled response;
tuning said response to control power absorption; and extracting the power absorbed.